Modern battery powered laptop computers such as computer 10 shown in FIG. 1A weigh less than perhaps 6 pounds (13.2 Kg) and may see use in environments ranging from humid rain forests to dusty deserts. Understandably, within computer 10 it is important that the keyboard switches 20, and other mechanical switches 30, e.g., the computer ON/OFF switch, be moisture and dust resistant. Shown in partial cutaway in FIG. 1A is keyboard switch 20', attached to the motherboard or other substrate 40 and surrounded by a dome-shaped typically rubber housing 50.
FIG. 1B is a detailed cross-sectional view of prior art mechanical switch 20', a switch commonly found in laptop computers and other equipment intended to function in wet, dusty, or other hostile environments. A mechanical switch mechanism 60 is attached to a motherboard or other substrate 40, and is surrounded by a flexible dome-shaped housing 50 that is sealingly attached with adhesive 70 or the like to the substrate. Mechanism 60 typically includes an actuator 80 that is biased with a spring 90. Upon activation by a downward force F upon an external keycap or button 100, spring-loaded actuator 80 moves downward a distance .DELTA.Y and makes electrical contact between first and second conductors 110, 120, one of which may be ground. As such, switch 20' changes from a first switching state (e.g., electrical "open") to a second switching state (e.g., electrical "short") between conductors 110 and 120.
Housing 50 should be flexible and should provide a hermetic seal that protects mechanism 90 from moisture, dust and the like. commonly, housing 50 is fabricated from rubber. But although switch actuator 90 may have a projected lifetime of a million operations or more, in practice the ability of housing 50 to seal against water and dust may be degraded after a few months. Repeated flexings, temperature extremes and general ageing can cause housing 50 to loose flexibility and wear out, developing cracks through which moisture and/or dust can penetrate. Further, the interface 70 between housing 50 and substrate 40 can deteriorate, providing further leakage paths into actuator 90. The unfortunate result can be intermittent functioning or even malfunctioning of switch 20'.
In addition to susceptibility to deterioration, the use of rubber or other non-conductive housing material 50 is detrimental in that it does not provide electrostatic shielding for the switch actuator 90 within. A user of computer 10 or other equipment associated with switch 20' may walk across a floor, building up static electricity that is discharged into equipment 10 when the user attempts to touch or use switch 20'.
Another undesirable aspect of prior art actuator switch mechanisms is the non-linear transfer function of user-generated actuation force (F) as a function of actuator travel (.DELTA.Y) that is created by the dome-shaped housing 50. FIG. 1C depicts the typically "S"-shaped transfer function curve that is characteristic of such assemblies. Initially, increasing F increases the travel distance .DELTA.Y but as the rubber dome 50 begins to compress downward, .DELTA.Y increases with decreasing force, until in a somewhat compressed configuration the dome again begins to exhibit a linear transfer function of F vs. .DELTA.Y. The S-shaped transfer function makes it difficult to design such switches to provide adequate tactile feedback for the user, while permitting efficient switch operation.
The transfer function characteristic is further complicated because the presence of a rubber housing requires even greater activation force, requiring the user to press harder to activate the switch mechanism. These requirements are especially important for laptop computer keyboard use, where actuator travel, actuation force, and tactile feedback should be predictable within a desired operating range
In summary, what is needed is a mechanical switch actuator that is moisture proof, and that can stand repeated operation. Such mechanism should also provide electrical shielding and a preferably substantially linear transfer function between actuation force and actuator travel. Preferably such actuator should not contribute substantial weight or cost to the laptop computer or other electronic device.
The present invention discloses such an actuator switch.